This invention relates to control circuits, and particularly to an arrangement for controlling an injection laser.
Use of an injection laser in an optical transmission system requires that the laser be prebiased with a current which is smaller than the laser threshold current (point above which the lasing action starts). This is necessary to minimize the delay between the input of an electrical signal to a laser and the laser corresponding light output. Because the laser threshold current varies with temperature and age, the prebias current has to be correspondingly adjusted for the proper operation of a laser in an optical transmission system.
Prior art laser control circuits are arranged to accomplish this by sensing the optical power output of a laser with a relatively slow feedback loop which averages the laser's optical power output over several time slots. A voltage derived from the optical power output is then compared to a fixed reference voltage by a difference amplifier which uses the difference to adjust its output and thus vary the prebias current. While such an arrangement provides an adequate adjustment of the prebias current to compensate for the threshold current variation, it has several shortcomings. When the density of light pulses in a laser light output varies, the prior art control circuit causes variations in the amplitudes of the individual light pulses. Furthermore, for an electrical input signal having a long string of digital zeroes or the removal of the input signal altogether, the prior art circuit causes the prebias current to increase so high that reapplication of the signal can cause irreversible damage to the laser.
Therefore, it is an object to provide an improved laser control circuit.
Another object is to provide a laser control circuit which maintains individual light pulses at a constant amplitude.
Yet another object is to provide a laser control circuit which prevents the laser from being irreversibly damaged in response to an electrical input signal.
These and other objects are realized in an illustrative emobodiment of the invention in which a control circuit for an injection laser includes a driver circuit which applies a driving current to the laser in response to an electrical modulating signal. A prebias circuit applies a prebias current to the laser in response to the difference between a signal derived from the electrical modulating signal and a signal derived from the laser light output.
A feature of the invention is the prebiasing of the laser in response to the difference between a signal derived from an electrical modulating signal and a signal derived from the laser light output.
Another feature of the invention is the prebiasing of the laser always below its threshold current.